Decaborane ionizer

Details Decaborane ionizer Decaborane ionizer

1999-10-11

2001-09-11

Berman, Jack (Department: 2881)

Radiant energy

Ion generation

Electron bombardment type

C250S492210

Reexamination Certificate

active

06288403

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to ion sources for ion implantation equipment and more specifically to an ion source for ionizing decaborane.
BACKGROUND OF THE INVENTION
Ion implantation has become a standard accepted technology of industry to dope workpieces such as silicon wafers or glass substrates with impurities in the large scale manufacture of items such as integrated circuits and flat panel displays. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.
Ion dose and energy are the two most important variables used to define an implant step. Ion dose relates to the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally greater than 10 milliamps (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable up to about 1 mA beam current) are used for lower dose applications. Ion energy is used to control junction depth in semiconductor devices. The energy of the ions which make up the ion beam determine the degree of depth of the implanted ions. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron volts (MeV), while shallow junctions may only demand energies below 1 thousand electron volts (keV).
The continuing trend to smaller and smaller semiconductor devices requires implanters with ion sources that serve to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy levels permit shallow implants. Source/drain junctions in complementary metal-oxide-semiconductor (CMOS) devices, for example, require such a high current, low energy application.
A typical ion source
10
for obtaining atoms for ionization from a solid form is shown in FIG.
1
. The ion source comprises a pair of vaporizers
12
and
14
and an ionization chamber
16
. Each of the vaporizers is provided with a crucible
18
in which a solid element or compound is placed and which is heated by a heater coil
20
to vaporize the solid source material. Heater coil leads
22
conduct electrical current to the heater coils and thermocouples
24
provide a temperature feedback mechanism. Air cooling conduit
26
and water-cooling conduit
28
is also provided.
Vaporized source material passes through a nozzle
30
, which is secured to the crucible
18
by a graphite nozzle retainer
32
, and through vaporizer inlets
34
to the interior of the ionization chamber
16
. Alternatively, compressed gas may be fed directly into the ionization chamber by means of a gas inlet
36
via a gas line
38
. In either case, the gaseous/vaporized source material is ionized by an arc chamber filament
40
that is heated to thermionically emit electrons.
Conventional ion sources utilize an ionizable dopant gas which is obtained either directly from a source of a compressed gas or indirectly from a solid which has been vaporized. Typical source elements are boron (B), phosphorous (P), gallium (Ga), indium (In), antimony (Sb), and arsenic (As). Most of these source elements are commonly used in both solid and gaseous form, except boron, which is almost exclusively provided in gaseous form, e.g., as boron trifluoride (BF
3
).
In the case of implanting boron trifluoride, a plasma is created which includes singly charged boron (B+) ions. Creating and implanting a sufficiently high dose of boron into a substrate is usually not problematic if the energy level of the beam is not a factor. In low energy applications, however, the beam of boron ions will suffer from a condition known as “beam blow-up”, which refers to the tendency for like-charged ions within the ion beam to mutually repel each other. Such mutual repulsion causes the ion beam to expand in diameter during transport, resulting in vignetting of the beam by multiple apertures in the beamline. This severely reduces beam transmission as beam energy is reduced.
Decaborane (B
10
H
14
) is a compound which is an excellent source of feed material for boron implants because each decaborane molecule (B
10
H
14
) when vaporized and ionized can provide a molecular ion comprised of ten boron atoms. Such a source is especially suitable for high dose/low energy implant processes used to create shallow junctions, because a molecular decaborane ion beam can implant ten times the boron dose per unit of current as can a monatomic boron ion beam. In addition, because the decaborane molecule breaks up into individual boron atoms of roughly one-tenth the original beam energy at the workpiece surface, the beam can be transported at ten times the energy of a dose-equivalent monatomic boron ion beam. This feature enables the molecular ion beam to avoid the transmission losses that are typically brought about by low energy ion beam transport.
However, decaborane ion sources to date have been unsuccessful at generating sufficient ion beam current for production applications of boron implants. Known hot-cathode sources are unsuitable for decaborane ionization because the heat generated by the cathode and arc in turn heats the walls and components to greater than 500° C., causing dissociation of the decaborane molecule into borane fragments and elemental boron. Known plasma-based sources are unsuitable for decaborane ionization because the plasma itself can cause dissociation of the decaborane molecule and fragmentation of the B
10
H
X
+
desired parent ion. Accordingly, in known decaborane ion sources, the source chamber pressure is kept sufficiently low to prevent the sustenance of a local plasma. Thus far, ion beam currents developed from such a source are too low for production applications.
Accordingly, it is an object of the present invention to provide an ion source for an ion implanter, which can accurately and controllably ionize sufficient decaborane to produce acceptable production ion beam current levels, to overcome the deficiencies of known ion sources.
SUMMARY OF THE INVENTION
An ion source for an ion implanter is provided, comprising a vaporizer and a remotely located ionizer connected to the vaporizer by a feed tube. The vaporizer comprises a sublimator for receiving a solid source material such as decaborane and sublimating (vaporizing) the decaborane. A heating mechanism is provided for heating the sublimator, and the feed tube connecting the sublimator to the ionizer, to maintain a suitable temperature for the vaporized decaborane.
The ionizer comprises a body having an inlet for receiving the vaporized decaborane; an ionization chamber in which the vaporized decaborane may be ionized by an energy-emitting element to create a plasma; and an exit aperture for extracting an ion beam comprised of the plasma. A cooling mechanism is provided for lowering the temperature of walls of the ionization chamber (e.g., to below 350° C.) during the ionization of the vaporized decaborane to prevent dissociation of vaporized decaborane molecules into atomic boron ions. In addition, the energy-emitting element is operated at a sufficiently low power level to minimize plasma density within the ionization chamber to prevent additional dissociation of the vaporized decaborane molecules by the plasma itself


REFERENCES:
patent: 5661308 (1997-08-01), Benveniste et al.
patent: 10-163123 (1998-06-01), None
K. Goto, J. Matsuo*, T. Sugii, H. Minakata, I. Yam

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